With the aid of modern imaging methods two- or three-dimensional image data is often produced which can be utilized for visualizing an imaged object under examination and in addition also for other applications.
The imaging methods are often based on the capture of X-ray radiation, where so-called projection measurement data is produced. For example, projection measurement data can be acquired with the aid of a computed tomography system (CT system). In CT systems a combination which is arranged on a gantry and which comprises an X-ray source and an oppositely arranged X-ray detector usually rotates around a measurement chamber in which the object under examination (which is referred to in the following without limiting the generality as the patient) is situated. In this situation the center of rotation (also referred to as “isocenter”) coincides with a so-called system axis z. During one or more rotations the patient is irradiated with X-ray radiation from the X-ray source, in which case projection measurement data or X-ray projection data which describes the X-ray attenuation of the patient in this direction of irradiation is captured with the aid of the oppositely arranged X-ray detector.
The projection measurement data produced, also referred to as projection data for short, is in particular dependent on the type of construction of the X-ray detector. X-ray detectors usually comprise a plurality of detection units which are mostly arranged in the form of a regular pixel array. The detection units in each case produce a detection signal for X-ray radiation impinging on the detection units which is analyzed at specific points in time in respect of intensity and spectral distribution of the X-ray radiation in order to provide information on the object under examination and to produce projection measurement data.
With certain types of CT imaging methods a plurality of imaging operations using X-ray radiation having different X-ray energy spectra are performed on one and the same field of examination of a patient. This process is also referred to as multi-energy CT imaging. Such multi-energy CT imaging can for example be performed with the aid of multiple successive CT imaging operations having a differing tube voltage. It is also possible to simultaneously implement imaging operations having differing energy spectra if an energy-sensitive detector is used and in the case of a single CT imaging operation X-ray attenuation data is recorded with differing effective spectra at the same time. This approach can for example be implemented with the aid of quantum-counting detectors or multi-layer detectors.
As a result of recording the same object using n differing energy spectra, where n is an integer, the amount of data is likewise increased in comparison with a single-energy CT imaging operation by the factor n. In consequence of the increased amount of data, in the case of multi-energy CT imaging operations problems can occur with regard to the transfer of data within a CT scanning unit. In addition, the long-term storage of the recorded data can be very complex.
Most known clinical evaluation methods are designed for dual-energy CT imaging operations, in other words CT imaging operations using only two differing X-ray energy spectra. A problem therefore consists in the fact that in the event of more than two energy spectra the existing methods can no longer be directly employed. For example, there are automated methods which use dual-energy CT image data sets as the basis for automatically differentiating kidney stone types or subtracting contrast agents from image data so that the body's own underlying tissue becomes visible. These applications cannot however be employed directly if more than two X-ray energy spectra have been used for the CT imaging operation.